This invention relates in general to fastener driving devices, and more specifically, to control circuits for electrically operated fastener driving devices such as for example electric tackers.
An example of an electrically operated tacker is shown in the commonly assigned U.S. patent application of Bernecki et al, Ser. No. 294,422, filed Aug. 19, 1981, now U.S. Pat. No. 4,417,681. That teachings of that application are hereby incorporated by reference into this application.
In order to positively prevent unwanted double firing, it is appropriate to energize the tacker's solenoid for no more than a single one-half cycle of commercially available AC power. Various circuits are known and used for this purpose, such as, for example, U.S. Pat. No. 3,215,864--Doyle et al (Nov. 2, 1965), the teachings thereof being incorporated herein by reference. In the Doyle et al circuit, a unidirectional controlled conduction device, such as a silicon controlled rectifier (SCR) is used to close a power circuit so that current will flow in an electrical load such as the tacker's solenoid winding. Current flow through the solenoid winding causes a mechanical power stroke to be produced for effecting a tacking operation.
Known control circuits generate a gate signal for triggering the SCR on the first properly poled half cycle of an applied AC voltage after the randomly timed actuation of a switch by a user. The control circuit also includes means for preventing the application of a further gate signal to the SCR until the switch is released and then re-actuated by the user.
These known actuating circuits use various schemes for generating the gate signal for triggering the SCR to "fire" the solenoid. For example, in the Doyle U.S. Pat. No. 3,215,864, a charge is built up on a capacitor. When the switch is actuated this charge is used to generate the gate signal. This is a rather typical approach to generating the firing signal. Such known circuits are, however, susceptible to misfirings (unintended firings) and are generally not immune to switch bounce, i.e., proper firing is inhibited when the actuating switch does not make a clean firing closure but instead, its switch contacts bounce open and close again one or more times before remaining closed.